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Toward developing an efficient plasma-based technique for CO2 reduction with CH4, warm-plasma catalytic (WPC) reduction of CO2 with CH4 in a reactor using gliding arc discharge and Ni/CeO2/Al2O3 catalyst is reported. In the warm plasma (WP) case, besides dry reforming reaction, reverse water gas shift (RWGS) reaction also comparably contributes for CO2 reduction. Although the contribution of dry reforming reaction is significantly enhanced by the increase of CH4/CO2 ratio, it accounts for only 60% due to the limitation of maximum CH4/CO2 ratio by the serious carbon deposition at CH4/CO2 >1/2. In addition, the low CH4/CO2 ratio also results in the low CO2 conversion, H2/CO ratio and H2 selectivity. In the WPC case, extra CH4 was fed after plasma to increase the CH4/CO2 ratio to 1, dry reforming reaction becomes the major contributor of CO2 reduction. Under the conditions of specific energy input of 86 kJ/mol, total flow rate of 2.67 SLM, catalyst-bed temperature of 850 °C and gas hourly space velocity of 3200 h−1, CO2 conversion of 91% and CH4 conversion of 94% with H2/CO = 1 and nearly 100% H2 selectivity are achieved with a CO2 energy cost of 598 kJ/mol and an energy efficiency of 72%.

In terms of the reaction of CO2 reduction to CO with hydrogen, CO2 conversion is very low at low temperature due to the limitation of thermodynamic equilibrium (TE). To overcome this limitation, plasma catalytic reduction of CO2 to CO in a catalyst-filled dielectric barrier discharge (DBD) reactor is studied. An enhanced effect of plasma on the reaction over Au/CeO2 catalysts is observed. For both the conventionally catalytic (CC) and plasma catalytic (PC, Pin = 15 W) reactions under conditions of 400 °C, H2/CO2 = 1, 200 SCCM, GHSV = 12,000 mL•g−1cat•h−1, CO2 conversions over Au/CeO2 reach 15.4% and 25.5% due to the presence of Au, respectively, however, those over CeO2 are extremely low and negligible. Moreover, CO2 conversion over Au/CeO2 in the PC reaction exceeds 22.4% of the TE conversion. Surface intermediate species formed on the catalyst samples during the reactions are determined by in-situ temperature-programmed decomposition (TPD) technique. Interestingly, it disclosed that in the PC reaction, the formation of formate intermediate is enhanced by plasma, and the acceleration by plasma in the decomposition of formate species is much greater than that in the formation of formate species on Au/CeO2. Enhancement factor is introduced to quantify the enhanced effect of plasma. Lower reactor temperature, higher gas hourly space velocity (GHSV), and lower molar ratio of H2/CO2 would be associated with larger enhancement factor.

CO 2 selective reduction to CO with H 2 over a CeO 2 -supported nano-Au catalyst at atmospheric pressure was investigated. A high CO 2 conversion, approaching the thermodynamic equilibrium value, and nearly 100% CO selectivity were obtained. The surface formate intermediates generated during the reverse water-gas shift reaction at 400℃ were identified using in situ diffuse-reflectance infrared Fourier-transform spectroscopy. The formate consumption to give CO and H 2 O, determined using mass spectrometry, indicated that the reaction proceeded via an associative formate mechanism; this contributes to the high Au/CeO 2 catalytic activity at low temperatures.